Global lighting effects the intensity of the lights you have in your scene. By changing the global and ambient values you can make you scene darker or lighter. I belief the ambient chauses to the scene to be darker, but i'm not sure.

I think you mean the AmbientOccluder attribute? This is all explained in an other topic created by steve. Short said you go into scene mode, creat a new attribute called 'AmbientOccluder'. then you make some in changes in the global lighting setup: uncheck project settings and make the values global:0 and ambient:3.14. Then render.

I belief, yet again not sure, go to scene mode add a new attribute called raydepth and change the int value. Standard raydepth is 13 so if you want shorter raydepth make it lower then 13 and visa versa with longer raydepth.